Prior art polycarbonates while possessing many excellent properties do have the tendency of discoloring upon exposure to heat. Numerous other possibilities for compensating the discoloration of polycarbonates on exposure to heat already exist, namely, for example, by adding additives, in particular phosphites (compare German Patent Specifications Nos. 1,128,653; 2,104,207, and 2,255,639 and their respective United States equivalents: U.S. Pat. Nos. 3,305,520; 3,794,629 and U.S. Patent application No. 635,010, filed Nov. 25, 1975 and now indicated to be allowed), optionally in combination with glycerol (German Patent Specification No. 2,039,971) or epoxides (German Patent Specification No. 2,402,367) or oxetane compounds (German Patent Specification No. 2,510,463 and its U.S. equivalent, U.S. Patent application No. 659,385, filed Feb. 19, 1976.
In contrast to these methods for stabilizing the color of polycarbonates towards exposure to heat, the method according to the invention uses a completely different principle, in that the tertiary amines to be employed as catalysts in the phase boundary polycondensation reaction, which are used, in particular, for the purpose of the condensation of the oligocarbonates which are primarily formed during the polycarbonate preparation by the phase boundary process and contain chlorocarbonic acid ester terminal groups, are distilled beforehand over reducing agents and/or acid anhydrides, with the exclusion of atmospheric oxygen.
The particular advantage of this process is that polycarbonates with a higher light transmission can be prepared without adding stabilizers, and this advantageously manifests itself when used continuously at relatively high temperatures on the one hand, or when used in the food sector on the other hand.